HYSTERESIS IN NITROGEN ADSORPTION-ISOTHERMS ON COPPER PHTHALOCYANINE PIGMENTS

The adsorption isotherms of nitrogen on copper phthalocyanine pigments often reveal hysteresis both at high and at low relative pressures. Two broad categories of hysteresis can be distinguished: in one category, the hysteresis loop broadens at relative pressures above about 0.4, and in the other it broadens towards lower pressures. An example is given of how the change from the first to the second category may occur when a copper phthalocyanine pigment is treated with an organic liquid. It is concluded that the first category of hysteresis is associated with pigment crystals aggregated into closely packed structures, whereas the second indicates more loosely packed aggregate structures.